Uncached buffered IO

Rather than pass in both the file and position directly from the kiocb, just take a struct kiocb instead. With the kiocb being passed in, skip passing in the address_space separately as well. While doing so, move the ki_flags checking into filemap_create_folio() as well. In preparation for actually needing the kiocb in the function.

No functional changes in this patch.

Signed-off-by Jens Axboe <[email protected]>

Rather than use the page_cache_sync_readahead() helper, define our own ractl and use page_cache_sync_ra() directly. In preparation for needing to modify ractl inside filemap_get_pages().

No functional changes in this patch.

Reviewed-by Christoph Hellwig <[email protected]>

Signed-off-by Jens Axboe <[email protected]>

Just a wrapper around filemap_alloc_folio() for now, but add it in preparation for modifying the folio based on the 'ractl' being passed in.

No functional changes in this patch.

Signed-off-by Jens Axboe <[email protected]>

Add a folio flag that file IO can use to indicate that the cached IO being done should be dropped from the page cache upon completion.

Signed-off-by Jens Axboe <[email protected]>

Quoted 4 lines · Jens Axboe

Add a folio flag that file IO can use to indicate that the cached IO being done should be dropped from the page cache upon completion.

Signed-off-by Jens Axboe <[email protected]>

Reviewed-by Kirill A. Shutemov <[email protected]>

+ David, Vlastimil.

I think we should consider converting existing folio_set_reclaim() / SetPageReclaim() users to the new flag. From a quick scan, all of them would benefit from dropping the page after writeback is complete instead of leaving the folio on the LRU.

Quoted 10 lines · Kirill A. Shutemov

Quoted 4 lines · Jens Axboe

Add a folio flag that file IO can use to indicate that the cached IO being done should be dropped from the page cache upon completion.

Signed-off-by Jens Axboe <[email protected]>

Reviewed-by Kirill A. Shutemov <[email protected]>

+ David, Vlastimil.

Quoted 4 lines

I think we should consider converting existing folio_set_reclaim() / SetPageReclaim() users to the new flag. From a quick scan, all of them would benefit from dropping the page after writeback is complete instead of leaving the folio on the LRU.

Ooh, that would be nice. Removes the overloading of PG_reclaim with PG_readahead, right?

Quoted 16 lines · Matthew Wilcox

Quoted 14 lines · Kirill A. Shutemov

Quoted 4 lines · Jens Axboe

Add a folio flag that file IO can use to indicate that the cached IO being done should be dropped from the page cache upon completion.

Signed-off-by Jens Axboe <[email protected]>

Reviewed-by Kirill A. Shutemov <[email protected]>

+ David, Vlastimil.

I think we should consider converting existing folio_set_reclaim() / SetPageReclaim() users to the new flag. From a quick scan, all of them would benefit from dropping the page after writeback is complete instead of leaving the folio on the LRU.

Quoted 2 lines

Ooh, that would be nice. Removes the overloading of PG_reclaim with PG_readahead, right?

Yep.

Quoted 4 lines · Jens Axboe

Add a folio flag that file IO can use to indicate that the cached IO being done should be dropped from the page cache upon completion.

Signed-off-by Jens Axboe <[email protected]>

Reviewed-by Kirill A. Shutemov <[email protected]>

+ David, Vlastimil.

I think we should consider converting existing folio_set_reclaim() / SetPageReclaim() users to the new flag. From a quick scan, all of them would benefit from dropping the page after writeback is complete instead of leaving the folio on the LRU.

Quoted 10 lines · Kirill A. Shutemov

Quoted 4 lines · Jens Axboe

Add a folio flag that file IO can use to indicate that the cached IO being done should be dropped from the page cache upon completion.

Signed-off-by Jens Axboe <[email protected]>

Reviewed-by Kirill A. Shutemov <[email protected]>

+ David, Vlastimil.

Quoted 4 lines

I think we should consider converting existing folio_set_reclaim() / SetPageReclaim() users to the new flag. From a quick scan, all of them would benefit from dropping the page after writeback is complete instead of leaving the folio on the LRU.

I wonder of there are some use cases where we write a lot of data to then only consume it read-only from that point on (databases? fancy AI stuff? no idea :) ).

Quoted 16 lines · David Hildenbrand

Quoted 14 lines · Kirill A. Shutemov

Quoted 4 lines · Jens Axboe

Add a folio flag that file IO can use to indicate that the cached IO being done should be dropped from the page cache upon completion.

Signed-off-by Jens Axboe <[email protected]>

Reviewed-by Kirill A. Shutemov <[email protected]>

+ David, Vlastimil.

I think we should consider converting existing folio_set_reclaim() / SetPageReclaim() users to the new flag. From a quick scan, all of them would benefit from dropping the page after writeback is complete instead of leaving the folio on the LRU.

Quoted 3 lines

I wonder of there are some use cases where we write a lot of data to then only consume it read-only from that point on (databases? fancy AI stuff? no idea :) ).

Do we use PG_reclaim for such cases?

If ractl->dropbehind is set to true, then folios created are marked as dropbehind as well.

Signed-off-by Jens Axboe <[email protected]>

mm/truncate.c +1 -10

-
-			if (folio_mapped(folio))
-				unmap_mapping_folio(folio);
-			BUG_ON(folio_mapped(folio));
-
-			ret2 = folio_launder(mapping, folio);
-			if (ret2 == 0) {
-				if (!invalidate_complete_folio2(mapping, folio))
-					ret2 = -EBUSY;
-			}
+			ret2 = folio_unmap_invalidate(mapping, folio, GFP_KERNEL);
 			if (ret2 < 0)
 				ret = ret2;
 			folio_unlock(folio);

mm/truncate.c int invalidate_inode_pages2_range(struct address_space *mapping, · line 644

Quoted 6 lines · Jens Axboe

mm/truncate.c +0 -1

@@ -629,18 +641,8 @@ int invalidate_inode_pages2_range(struct address_space *mapping,
 				folio_unlock(folio);
 				continue;
 			}
-			VM_BUG_ON_FOLIO(!folio_contains(folio, indices[i]), folio);
 			folio_wait_writeback(folio);

Any particular reason you drop this VM_BUG_ON_FOLIO()?

Hi,

5 years ago I posted patches adding support for RWF_UNCACHED, as a way to do buffered IO that isn't page cache persistent. The approach back then was to have private pages for IO, and then get rid of them once IO was done. But that then runs into all the issues that O_DIRECT has, in terms of synchronizing with the page cache.

So here's a new approach to the same concent, but using the page cache as synchronization. Due to excessive bike shedding on the naming, this is now named RWF_DONTCACHE, and is less special in that it's just page cache IO, except it prunes the ranges once IO is completed.

Why do this, you may ask? The tldr is that device speeds are only getting faster, while reclaim is not. Doing normal buffered IO can be very unpredictable, and suck up a lot of resources on the reclaim side. This leads people to use O_DIRECT as a work-around, which has its own set of restrictions in terms of size, offset, and length of IO. It's also inherently synchronous, and now you need async IO as well. While the latter isn't necessarily a big problem as we have good options available there, it also should not be a requirement when all you want to do is read or write some data without caching.

Even on desktop type systems, a normal NVMe device can fill the entire page cache in seconds. On the big system I used for testing, there's a lot more RAM, but also a lot more devices. As can be seen in some of the results in the following patches, you can still fill RAM in seconds even when there's 1TB of it. Hence this problem isn't solely a "big hyperscaler system" issue, it's common across the board.

Common for both reads and writes with RWF_DONTCACHE is that they use the page cache for IO. Reads work just like a normal buffered read would, with the only exception being that the touched ranges will get pruned after data has been copied. For writes, the ranges will get writeback kicked off before the syscall returns, and then writeback completion will prune the range. Hence writes aren't synchronous, and it's easy to pipeline writes using RWF_DONTCACHE. Folios that aren't instantiated by RWF_DONTCACHE IO are left untouched. This means you that uncached IO will take advantage of the page cache for uptodate data, but not leave anything it instantiated/created in cache.

File systems need to support this. This patchset adds support for the generic read path, which covers file systems like ext4. Patches exist to add support for iomap/XFS and btrfs as well, which sit on top of this series. If RWF_DONTCACHE IO is attempted on a file system that doesn't support it, -EOPNOTSUPP is returned. Hence the user can rely on it either working as designed, or flagging and error if that's not the case. The intent here is to give the application a sensible fallback path - eg, it may fall back to O_DIRECT if appropriate, or just live with the fact that uncached IO isn't available and do normal buffered IO.

Adding "support" to other file systems should be trivial, most of the time just a one-liner adding FOP_DONTCACHE to the fop_flags in the file_operations struct.

Performance results are in patch 8 for reads, and you can find the write side results in the XFS patch adding support for DONTCACHE writes for XFS:

://git.kernel.dk/cgit/linux/commit/?h=buffered-uncached.9&id=edd7b1c910c5251941c6ba179f44b4c81a089019

with the tldr being that I see about a 65% improvement in performance for both, with fully predictable IO times. CPU reduction is substantial as well, with no kswapd activity at all for reclaim when using uncached IO.

Using it from applications is trivial - just set RWF_DONTCACHE for the read or write, using pwritev2(2) or preadv2(2). For io_uring, same thing, just set RWF_DONTCACHE in sqe->rw_flags for a buffered read/write operation. And that's it.

Patches 1..7 are just prep patches, and should have no functional changes at all. Patch 8 adds support for the filemap path for RWF_DONTCACHE reads, and patches 9..11 are just prep patches for supporting the write side of uncached writes. In the below mentioned branch, there are then patches to adopt uncached reads and writes for xfs, btrfs, and ext4. The latter currently relies on bit of a hack for passing whether this is an uncached write or not through ->write_begin(), which can hopefully go away once ext4 adopts iomap for buffered writes. I say this is a hack as it's not the prettiest way to do it, however it is fully solid and will work just fine.

Passes full xfstests and fsx overnight runs, no issues observed. That includes the vm running the testing also using RWF_DONTCACHE on the host. I'll post fsstress and fsx patches for RWF_DONTCACHE separately. As far as I'm concerned, no further work needs doing here.

And git tree for the patches is here:

https://git.kernel.dk/cgit/linux/log/?h=buffered-uncached.9

 include/linux/fs.h             | 21 +++++++-
 include/linux/page-flags.h     |  5 ++
 include/linux/pagemap.h        |  1 +
 include/trace/events/mmflags.h |  3 +-
 include/uapi/linux/fs.h        |  6 ++-
 mm/filemap.c                   | 97 +++++++++++++++++++++++++++++-----
 mm/internal.h                  |  2 +
 mm/readahead.c                 | 22 ++++++--
 mm/swap.c                      |  2 +
 mm/truncate.c                  | 54 ++++++++++---------
 10 files changed, 166 insertions(+), 47 deletions(-)

Since v6
- Rename the PG_uncached flag to PG_dropbehind
- Shuffle patches around a bit, most notably so the foliop_uncached patch goes with the ext4 support
- Get rid of foliop_uncached hack for btrfs (Christoph)
- Get rid of passing in struct address_space to filemap_create_folio()
- Inline invalidate_complete_folio2() in folio_unmap_invalidate() rather than keep it as a separate helper
- Rebase on top of current master

Signature · 1 line

Jens Axboe

Quoted 104 lines · Jens Axboe

Hi,

5 years ago I posted patches adding support for RWF_UNCACHED, as a way to do buffered IO that isn't page cache persistent. The approach back then was to have private pages for IO, and then get rid of them once IO was done. But that then runs into all the issues that O_DIRECT has, in terms of synchronizing with the page cache.

So here's a new approach to the same concent, but using the page cache as synchronization. Due to excessive bike shedding on the naming, this is now named RWF_DONTCACHE, and is less special in that it's just page cache IO, except it prunes the ranges once IO is completed.

Why do this, you may ask? The tldr is that device speeds are only getting faster, while reclaim is not. Doing normal buffered IO can be very unpredictable, and suck up a lot of resources on the reclaim side. This leads people to use O_DIRECT as a work-around, which has its own set of restrictions in terms of size, offset, and length of IO. It's also inherently synchronous, and now you need async IO as well. While the latter isn't necessarily a big problem as we have good options available there, it also should not be a requirement when all you want to do is read or write some data without caching.

Even on desktop type systems, a normal NVMe device can fill the entire page cache in seconds. On the big system I used for testing, there's a lot more RAM, but also a lot more devices. As can be seen in some of the results in the following patches, you can still fill RAM in seconds even when there's 1TB of it. Hence this problem isn't solely a "big hyperscaler system" issue, it's common across the board.

Common for both reads and writes with RWF_DONTCACHE is that they use the page cache for IO. Reads work just like a normal buffered read would, with the only exception being that the touched ranges will get pruned after data has been copied. For writes, the ranges will get writeback kicked off before the syscall returns, and then writeback completion will prune the range. Hence writes aren't synchronous, and it's easy to pipeline writes using RWF_DONTCACHE. Folios that aren't instantiated by RWF_DONTCACHE IO are left untouched. This means you that uncached IO will take advantage of the page cache for uptodate data, but not leave anything it instantiated/created in cache.

File systems need to support this. This patchset adds support for the generic read path, which covers file systems like ext4. Patches exist to add support for iomap/XFS and btrfs as well, which sit on top of this series. If RWF_DONTCACHE IO is attempted on a file system that doesn't support it, -EOPNOTSUPP is returned. Hence the user can rely on it either working as designed, or flagging and error if that's not the case. The intent here is to give the application a sensible fallback path - eg, it may fall back to O_DIRECT if appropriate, or just live with the fact that uncached IO isn't available and do normal buffered IO.

Adding "support" to other file systems should be trivial, most of the time just a one-liner adding FOP_DONTCACHE to the fop_flags in the file_operations struct.

Performance results are in patch 8 for reads, and you can find the write side results in the XFS patch adding support for DONTCACHE writes for XFS:

://git.kernel.dk/cgit/linux/commit/?h=buffered-uncached.9&id=edd7b1c910c5251941c6ba179f44b4c81a089019

with the tldr being that I see about a 65% improvement in performance for both, with fully predictable IO times. CPU reduction is substantial as well, with no kswapd activity at all for reclaim when using uncached IO.

Using it from applications is trivial - just set RWF_DONTCACHE for the read or write, using pwritev2(2) or preadv2(2). For io_uring, same thing, just set RWF_DONTCACHE in sqe->rw_flags for a buffered read/write operation. And that's it.

Patches 1..7 are just prep patches, and should have no functional changes at all. Patch 8 adds support for the filemap path for RWF_DONTCACHE reads, and patches 9..11 are just prep patches for supporting the write side of uncached writes. In the below mentioned branch, there are then patches to adopt uncached reads and writes for xfs, btrfs, and ext4. The latter currently relies on bit of a hack for passing whether this is an uncached write or not through ->write_begin(), which can hopefully go away once ext4 adopts iomap for buffered writes. I say this is a hack as it's not the prettiest way to do it, however it is fully solid and will work just fine.

Passes full xfstests and fsx overnight runs, no issues observed. That includes the vm running the testing also using RWF_DONTCACHE on the host. I'll post fsstress and fsx patches for RWF_DONTCACHE separately. As far as I'm concerned, no further work needs doing here.

And git tree for the patches is here:

https://git.kernel.dk/cgit/linux/log/?h=buffered-uncached.9

 include/linux/fs.h             | 21 +++++++-
 include/linux/page-flags.h     |  5 ++
 include/linux/pagemap.h        |  1 +
 include/trace/events/mmflags.h |  3 +-
 include/uapi/linux/fs.h        |  6 ++-
 mm/filemap.c                   | 97 +++++++++++++++++++++++++++++-----
 mm/internal.h                  |  2 +
 mm/readahead.c                 | 22 ++++++--
 mm/swap.c                      |  2 +
 mm/truncate.c                  | 54 ++++++++++---------
 10 files changed, 166 insertions(+), 47 deletions(-)

Quoted 14 lines

Since v6
- Rename the PG_uncached flag to PG_dropbehind
- Shuffle patches around a bit, most notably so the foliop_uncached patch goes with the ext4 support
- Get rid of foliop_uncached hack for btrfs (Christoph)
- Get rid of passing in struct address_space to filemap_create_folio()
- Inline invalidate_complete_folio2() in folio_unmap_invalidate() rather than keep it as a separate helper
- Rebase on top of current master

Signature · 1 line

Jens Axboe

Hi Jens,

You may recall I tested NFS to work with UNCACHED (now DONTCACHE). I've rebased the required small changes, feel free to append this to your series if you like.

More work is needed to inform knfsd to selectively use DONTCACHE, but that will require more effort and coordination amongst the NFS kernel team.

Since v6
- Rename the PG_uncached flag to PG_dropbehind
- Shuffle patches around a bit, most notably so the foliop_uncached patch goes with the ext4 support
- Get rid of foliop_uncached hack for btrfs (Christoph)
- Get rid of passing in struct address_space to filemap_create_folio()
- Inline invalidate_complete_folio2() in folio_unmap_invalidate() rather than keep it as a separate helper
- Rebase on top of current master

Quoted 9 lines

Since v6
- Rename the PG_uncached flag to PG_dropbehind
- Shuffle patches around a bit, most notably so the foliop_uncached patch goes with the ext4 support
- Get rid of foliop_uncached hack for btrfs (Christoph)
- Get rid of passing in struct address_space to filemap_create_folio()
- Inline invalidate_complete_folio2() in folio_unmap_invalidate() rather than keep it as a separate helper
- Rebase on top of current master

Hm. v6 had a patch that cleared the PG_uncached flag if the page accessed via non-uncached lookup[1]. What happened to it? I don't see it here.

https://lore.kernel.org/all/[email protected]